This invention relates to a process for the liquefaction of starch in the manufacture of a maltose-containing substance by the saccharification of a liquid starch solution.
One known method heretofore adopted for the manufacture of a maltose-containing substance from starch has comprised the steps of saccharifying a liquid starch solution with a saccharifying enzyme, optionally treating the saccharified starch solution further with .alpha.-amylase added thereto, refining the saccharified solution through filtration, a treatment with an activated carbon and a treatment with an ion-exchange resin, and concentrating the refined solution thereby producing a maltose-containing solution. When necessary, this solution was further concentrated and dried to produce powdered maltose. More recently, there has been proposed a method for producing maltose in high yields by treating the liquid starch solution with .beta.-amylase and .alpha.-1,6-glucosidase (U.S. Pat. No. 3,992,261).
The methods heretofore proposed for the liquefaction of starch include (i) a method which involves heating starch milk at a temperature in the range of from 150.degree. to 165.degree. C., with the pH value of the starch milk adjusted, or not adjusted, in the range of from 5 to 6 in advance, (ii) a method which involves maintaining starch milk in the presence of an acid added thereto at 120.degree. C. for a short period of time, (iii) a method which involves adjusting the pH value of starch in the range of from 5 to 6, adding .alpha.-amylase to the starch milk, heating the resultant mixture at a temperature in the range of from 80.degree. to 95.degree. C. for 1 to 30 minutes and thereafter maintaining the mixture at a temperature in the range of from 120.degree. to 140.degree. C. for 5 to 20 minutes thereby inactivating the .alpha.-amylase, and (iv) a method which, subsequently to the procedure of the method (iii) terminated by the step of the inactivation of .alpha.-amylase, further involves cooling the heated mixture to 90.degree. C., thereafter adding .alpha.-amylase again to the mixture, and effecting secondary liquefaction, for example.
In the methods of liquefaction described above, even when the adjustment of the pH value of starch milk is completely omitted, the starch for saccharification to be used on a commercial scale contains some amount of an acidic substance. When this starch is suspended in water, the resultant suspension has its pH value gradually fall with elapse of time and eventually reaches a point of exhibiting acidity. In the case of corn starch, for example, the SO.sub.2 which has been used in the course of the production of corn starch persists in the corn starch and constitutes itself one main cause for the drop of the pH value of the corn starch during the liquefaction and saccharification of the starch. In the case of sweet potato starch, the starch is recovered first in the form of starch milk from the sweet potatoes and the starch milk thus recovered is subjected in its unmodified form to starch saccharification or subjected to drying by dehydration. In any event, in some if not all of the lots, the starch may be immersed in water. During such prolonged standing under water, the starch may possibly undergo fermentation and produce organic acids such as lactic acid and butyric acid, and thereafter entrain such organic acids. In this respect, the commercial-grade starch is different from the refined starch of the reagent grade. Worse still, in the case of a method which involves preparatory adjustment of the pH value in the range of from 5.5 to 6.0 or a method which effects the liquefaction by addition of an acid, the pH value continues to drop with the gradual progress of liquefaction and reaches a very low level by the time that the liquefaction is completed. This adverse situation is generally corrected by readjusting the reaction system to the optimum pH value for the saccharifying enzyme either before or during the saccharification.
As the source for starch, aerially grown starches such as corn, wheat, and sago palm and underground grown starches such as potato, sweet potato, and cassava are available. It is known that generally the aerially grown starches contain sparingly soluble starches in higher percentages than the underground grown starches and, therefore, undergo liquefaction with greater difficulty. According to the passages found on pages 123-129 and pages 314-319 of "Agricultural and Biological Chemistry" (Agr. Biol. chem.), Vol. 32, 1968, insoluble starch is defined as what is obtained by liquefying a given starch with an ample amount of bacterial .alpha.-amylase, collecting the residual insoluble component by centrifugal separation, and assaying the insoluble component by the anthrone method. According to the test results reported, the insoluble starch content is about 230 mg% in corn starch, more than 200 mg% in wheat starch, about 9 mg% in potato starch, and about 25 mg% in sweet potato starch. (Here, "mg%" is used to signify the weight (in mg) of insoluble starch per 100 g of a given starch.)
An effort made to obtain from a starch having a high insoluble starch content a liquid starch solution having a low hydrolysis ratio (such as less than DE 2, for example) necessary for the production of a saccharified product of a high maltose content (such as not less than 80% of solids, for example) results in production of a cloudy white liquid starch solution with residual unliquefied starch particles in suspension. Consequently, the solution after the saccharification is difficult to refine. On the other hand, an effort to obtain a liquid starch solution free from such a suspension as mentioned above results in production of a solution involving excessive hydrolysis of starch. The liquid starch solution, when saccharified, does not give a product of improved maltose purity. In the case of the aforementioned known method of liquefaction, the pH value is low at the end of the liquefaction. In the case of the saccharifying enzyme to be used in the present invention, sine the optimum pH value falls in the range of from 6 to 8, it becomes frequently necessary to have the pH value of the liquid starch solution readjusted prior to and during the reaction of saccharification. The necessity for the pH value readjustment has proved to be a problem from the standpoint of prevention of contamination and of operational efficiency.